Anaerobic fermentations of hydrogen and carbon monoxide involve the contact of the substrate gas in an aqueous fermentation menstruum with microorganisms capable of generating alcohols such as ethanol, propanol, i-butanol and n-butanol. The production of these alcohols requires significant amounts of hydrogen and carbon dioxide and/or carbon monoxide. For instance, the theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O.C2H5OH+4CO2 6H2+2CO2.C2H5OH+3H2O.
As can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion. For purposes herein, it is referred to as the hydrogen conversion.
Typically the substrate gas for carbon monoxide and hydrogen conversions is, or is derived from, a synthesis gas (syngas) from the gasification of carbonaceous materials, partial oxidation or reforming of natural gas and/or biogas from anaerobic digestion or landfill gas or off-gas streams of various industrial methods such as off gas from coal coking and steel manufacture. The substrate gas contains carbon monoxide, hydrogen, and carbon dioxide and usually contains other components such as water vapor, nitrogen, methane, ammonia, hydrogen sulfide and the like.
These anaerobic fermentation processes are suitable for continuous processes. The syngas is passed into a bioreactor the aqueous fermentation broth for the bioconversion. Off gases can be removed from the bioreactor, and aqueous broth can be withdrawn from the bioreactor for recovery of the oxygenated organic compound at a rate sufficient to maintain steady-state operation. For such processes to be commercially viable they must be able to benefit from the advantages of scale, and thus facilities using these processes need to be able to produce upwards of 50 or 100 million or more gallons of oxygenated organic compound per year. These anaerobic fermentation processes necessarily involve the mass transfer of substrate from the gas phase into the liquid phase for access by the microorganisms. These mass transfer considerations together with economies of scale, tend to favor the use of large reactors for commercial-scale facilities. Hence, commercial scale reactors, i.e., those with liquid capacities of at least 1 million, and more often at least about 5, say, 5 to 25, million, liters would be advantageous.
The start-up of these commercial-scale facilities can be problematic due to the large volume of microorganisms required and the time required to grow a sufficient population of the microorganisms. The microorganisms for the anaerobic fermentation typically are expected to be generated by seed farms at the site of the facility. The capital and operating expense for a seed farm is not insignificant. Usually the seed farms are comprised of a sequential series of reactors of increasing size with the final reactor having enough volume to provide an initial charge to the commercial-scale reactor. Usually, the growth in each seed farm stage is targeted to increase the size of the population by a factor of 10 and each stage usually takes from 2 to 7 days to achieve the sought growth. Once charged from the seed farm, the reactor is then operated to promote the growth of the population of microorganisms while increasing the volume of the aqueous medium in the reactor until steady-state is achieved. U.S. Published Patent Application 20130078693 discloses processes for starting up and operation of deep tank anaerobic fermentation reactors.
The supply of syngas is subject to disruptions, both planned and unplanned. The microorganisms used for the bioconversion of syngas to alcohol have a limited period where viability can be retained after a cessation of flow of syngas. Under typical temperatures used for the bioconversion, the microorganisms quickly lose viability, and a loss of syngas for a period of 24 hours will result in at least about 90 percent of the population of microorganisms being killed. Reestablishing the population of microorganisms after such a decrease or cessation of syngas flow requires time, and during this time alcohol is not being produced at the sought rates. Thus, it is important for a commercial-scale facility to be able to substantially maintain as viable the population of microorganisms during any period where the syngas feed is materially decreased or ceased. Any method for maintaining the viability of the population of microorganisms should be effective for at least the most frequent duration of impaired syngas supply, which is typically at least about 6 hours, and more often at least about 12 to 24 hours, and potentially for several days to a week. It is axiomatic that any such method be able to be quickly implemented to minimize damage to the population of microorganisms. Moreover, the method itself should not induce unduly adverse effects on the microorganisms. Further, the method should not unduly hinder resumption of, or otherwise adversely affect, the normal operations once the impairment of the syngas supply has been alleviated, and the method should be economically viable to implement on a commercial-scale facility.
One option is to introduce sugar into the fermentation broth as substrate for the microorganisms in the event of a syngas feed interruption. This option would increase the risk of microbial contamination since it provides an environment conducive to the growth of a wide variety of microorganisms. It also would result in the generation of free (un-ionized) acid that requires the addition of alkalinity to avoid killing the microorganisms. For instance, one mole of fructose would yield three moles of acetic acid.
Adams, et al., in United States Patent Application Publication 2010/0227377 A1 propose adding carbon dioxide during periods of decrease or ceased syngas flow to a fermentation broth used to produce ethanol. They postulate that carbon dioxide and ethanol serve to provide energy back to the culture to maintain viability. The method disclosed by Adams, et al., is not without challenges. First, the method requires the introduction of carbon dioxide into the fermentation reactor. The dissolved concentration is dependent upon the gas transfer rate and uptake by the microorganisms and thus is difficult to control. Second, the metabolic reaction results in the production of acetic acid. This can result in a significant accumulation of free (un-ionized) acids in the fermentation reactor. The acidity must therefore be addressed by the addition of an alkalinity source, but the build-up of the cation associated with the alkalinity source can reach inhibitory levels. Moreover, the patent applicants do not disclose methods for maintaining the redox potential of the fermentation broth suitable for restart of the syngas fermentation once the flow of syngas can be restored.
Frostl, et al, in “Effect of nitrate on the autotrophic metabolism of the acetogens Clostridium thermoautotrophicum and Clostridium thermoaceticum, J. of Bacteriology, 1996, 178(15), pages 4597 to 4603, demonstrated the effect of nitrate anion on the growth and product profiles of the subject acetogens in the presence of a carbon dioxide feed. A variety of substrates were used by the authors with and without nitrate anion. Their summary states in part:                “Although nitrate stimulated the capacity of Clostridium thermoautotrophicum and Clostridium thermoaceticum to oxidize (utilize) substrates under heterotrophic conditions, it inhibited autotrophic H2—CO2-dependent growth.” . . . . “Nitrate has no appreciable effect on the specific activities of enzymes central to the acetyl-coenzyme A (CoA) pathway. However, membranes obtained from cells cultivated under nitrate-dissimilating conditions were deficient in the b-type cytochrome that was typical of membranes from acetogenic cells, i.e., cells dependent upon the synthesis of acetate for the conservation of energy.” . . . .        
Accordingly, improved methods are sought to maintain a viable microorganism population in a fermentation broth during periods of decreased or ceased syngas feed.